The requirement for prosthetic treatments of long duration (implants or knee, hip, maxillofacial, cranial, etc. prostheses), is increasingly usual in daily clinical practice, and involves the use, in numerous cases, of metallic materials (subcutaneous or osseous implants) mainly in patients subjected to major traumatic surgery, maxillofacial surgery, osteoporotic and osteoproliferative patients. The employment of these systems of prosthetic substitution is accompanied by a not insignificant implant failure rate, which in some cases surpasses 30%, and at times makes recourse to this technique an impossibility.
The most frequent complications encountered, described in the medical literature, are the infectious type (infection of the implant, bacteremia, sepsis, and others less frequent, like gangrene, etc.), the inflammatory type (reaction to a foreign body, local inflammation, total rejection), those of tissue integration (gingivitis, sinovial metallosis, osteoresorption) and those arising from their handling and use (rupture of the bone, failure of the metal-tissue interface).
The biocompatible metals constitute the most important and diverse group of materials used in biomedical applications because they offer appropriate properties of biocompatibility and chemical inertia which make them suitable for contact with biological fluids and tissues. Also, they have the characteristic that they can be manufactured in a great variety of ways.
However, the evolution observed in recent years regarding the types of alloys employed has not reduced the number of complications as much as would have been expected and the experimental procedures used to improve their biocompatibility have been limited to more permeable designs or surface impregnations, more or less intense, with molecules having biological activity (antibiotics, antiseptics, antiaggregants, etc.).
The requirement remains to develop medical materials whose employment permits avoidance of the entirety or part of the aforementioned complications.
In that concerning the problem of osseointegration of the prostheses or implants, a method of approaching the problem could consist in applying a surface treatment thereon which confers upon them the appropriate characteristics.
This is the case of ion implantation, a treatment which does not modify the structural properties or the dimensional tolerances of the treated prostheses or implants (see FIG. 1) but which, however, can modify their surface properties by means of the introduction of a series of selected elements on the surface, modifying the properties thereof in the desired sense.
Use has been made of different techniques of ion implantation for many years in different fields of application with the object of modifying the surface properties of the components. It is used, for example, in electronics for modification of the electrical properties of semiconductors. It is also applied in the metal mechanics industry for the improvement of properties of resistance to abrasion and corrosion, in cases such as moulds and injection mouthpieces, machining and cutting tools, gauges, etc.
Ion implantation has also been used on biomaterials. This is the case, for example, of the implantation of germicidal elements in medical equipment described in U.S. Pat. No. 5,492,763, or the implantation in implants of cobalt-chromium alloys with the object of increasing surface hardness and reducing friction as described in European patent application EP 526 581. The problem of osseointegration has also been broached from the ion implantation technique in order to produce a surface coated with hydroxyapatite, a coating which has also been applied by other processes. Such is the case of the method for the production of surgical implantations coated with synthetic bone described in Spanish patent ES 2.006.658 which employs high energy streams of xenon to coat the implants with hydroxyapatite by the sputtering or cathodic spraying technique. German patent application DE 19830530 describes the production of titanium surfaces coated with calcium phosphate by ion implantation. In this last case, use is made of phosphorus and calcium implantation followed by a heat treatment.
Notwithstanding the existence of previous applications of ion implantation in implants and medical prostheses, the ion implantation methods employed provide implants and prostheses with insufficient osseointegration properties and/or with a risk of lixiviation of the ions to the physiologic medium in contact with the inadequate implants and prostheses, and/or with not completely satisfactory tribological properties.